Ha! I asked this very same question last week sometime. Or maybe two weeks ago.
Either way, I was reprimanded for not searching first, but helpfully (and politely!) given this link:
http://www.linux-sxs.org/upgrading/migration.html
Haven't bothered with it yet (bad lazy me... too many other things to distract my attention), but it looks right enough. Not that I would know, but it's fun to guess...